This invention relates to a process for the purification of wastewater wherein the wastewater is first introduced into an aerobic biological treatment stage and subsequently into a precipitation stage.
Organically polluted wastewaters are usually subjected to biological purification. After the biological purification, substances which cannot be biograded, or are biogradable only with difficulties, remain in the wastewater and require removal. This is a particularly important problem in the case of certain industrial effluents, for example cellulose wastewaters. To remove these substances which are refractory to biological purification, a downstream precipitation stage is employed to occlude and/or adsorb the substances, for example by precipitating with Fe salts, Al salts, or bentonite, etc.
Classical methods employing a precipitation stage following a biological stage exhibit the drawback that precipitation chemicals are costly and cannot be recovered, thereby adding a significant cost to the system. Moreover, large amounts of organically loaded inorganic precipitation sludge must be dumped, thereby causing secondary expenses and problems.